X-ray imaging systems typically include an X-ray apparatus operable to generate a beam of X-rays, a detection apparatus, and a control system connected to the X-ray apparatus and detection apparatus. The X-ray apparatus produces a beam of X-rays which interact with a subject and are detected by operation of the detection apparatus. One typical example of an X-ray imaging system is a high performance computed tomography (CT) X-ray imaging system, which accommodates a human subject for medical imaging. Medical X-ray imaging systems typically include a gantry which is movable in relation to the human subject.
X-ray apparatus typically include an X-ray tube which is operable to generate a beam of X-rays. A typical X-ray tube includes a housing which forms an evacuated chamber. The housing supports inside the chamber a cathode assembly with a cathode filament. A high voltage electrical circuit is formed between the cathode and an anode assembly supported inside the housing. The anode assembly includes an X-ray target spaced from the cathode filament. The X-ray target includes a generally disk-shaped target cap. The target cap is formed of a high conductivity refractory metal, such as an alloy of molybdenum. An annular focal track on the front surface of the target cap includes a suitable X-ray emitting material, such as a chemical species of high atomic weight, of a type which interacts with high energy electrons to emit X-rays. The X-ray target also includes a heat sink affixed to a rear surface of the target cap. The heat sink receives intense heat conducted away from the focal track and substrate. Typically, the heat sink is formed of an annular block of graphite brazed to the rear surface of the target cap. The target cap is supported for rotation about a longitudinal axis. High speed rotation of the X-ray target is driven by a rotor connected to a drive motor.
For an imaging scan, the electrical circuit energizes the cathode filament to generate high energy electrons which impinge upon the focal track of the X-ray target. Interactions between the electrons and high atomic weight species in the focal track emit high frequency electromagnetic waves, or X-rays. X-rays directed through a window in the chamber housing are focused on a subject for imaging purposes. The electron interactions release intense heat into the focal track and target cap. The X-ray target is rotated by the motor at high speed in order to avoid overheating. Heat is also conducted out of the focal track into the substrate, and then into the heat sink. Heat dissipates from the heat sink through evacuated space in the chamber and into the housing. The housing is cooled by immersion in an external fluid bath.
Conventional X-ray targets presently possess material densities ranging from about 90.0% to about 95.0% of theoretical density. X-ray targets possessing material densities ranging from about 90.0% to about 95.0% of theoretical density are hindered by remaining porosity and porosity variation. X-ray targets can be produced by a “PSF” method by cold pressing (P) a form of substrate material and X-ray emitting material, sintering (S) the cold pressed form, and forging (F) the sintered form to desired shape. X-ray targets produced by the PSF method can possess material densities ranging from about 90.0% to about 95.0% of theoretical density. X-ray targets produced by the PSF method can be hindered by limited density, density variations, remaining porosity, porosity variations, limited mechanical strength properties, variation of mechanical strength properties, limited thermal conductivity, limited thermo-mechanical properties, limited thermal loading capacity, limited mechanical loading capacity. Examples of specific properties limited by the foregoing include: resistance to creep, tensile strength, compressive strength, thermal conductivity, bulk modulus, yield strength, mass per unit diameter, X-ray target diameter, thermal durability per unit of mass, mechanical durability per unit of mass, fatigue resistance, resistance to fatigue crack growth, resistance to crack growth, focal track life, and focal track performance. X-ray apparatus including X-ray targets having the foregoing limitations are hindered by limited capacity to operate at peak power, limited X-ray target rotation speed, limited gantry rotation speed, limited X-ray output at peak power, limited frequency of exposures at peak power, longer cooling periods between exposures, and limited cycle rate.
The specified limitations of X-ray targets produced by the PSF method can worsen as diameter of the X-ray target increases. Targets produced by the PSF method can suffer CTE mismatched bending stress or warpage because of differences between material properties of the focal track and the substrate material supporting the focal track. X-ray targets produced by the PSF method are hindered by the limitation that microstructure of the substrate and focal track materials is not highly controlled and, thus, variations of material properties such as microstructure and variation of microstructure are not optimal and are subject to great variation.
For reasons stated above and for other reasons which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for improved X-ray targets, X-ray apparatus, and X-ray imaging systems, and for improved methods of manufacturing the same.